Optical fiber connectors are a critical part of many different light transmission systems. For example, they are found in essentially all optical fiber communication systems and they are used in imaging light onto targets in military applications. Such connectors may be used to join segments of fiber into longer lengths, to connect fiber to active devices, such as radiation sources, detectors and repeaters, and to connect fiber to passive devices, such as switches, multiplexers, and attenuators. The principal function of an optical fiber connector is to hold the fiber end such that the fiber's core is axially aligned with an optical pathway of the mating structure. This way, light from the fiber is optically coupled to the optical pathway.
Of particular interest herein are “expanded beam” optical connectors. Such connectors are used traditionally in high vibration, high power and/or dirty environments, where “physical contact” between the fiber and the light path of a mating connector is problematic. To avoid problems of debris and vibration, expanded beam optical connectors have been developed which expand the optical beam and transmit it over an air gap between the connectors. By expanding the beam, its relative size increases with respect to the debris, making it less susceptible to interference. Further, transmitting the beam over an air gap eliminates component-to-component wear, thereby increasing the connector's endurance to vibration. Over the years, the expanded beam connector has evolved into a ruggedized multi-fiber connector comprising an outer housing which is configured to mate with the outer housing of a mating connector, typically through a screw connection. Contained within the outer housing are a number of inner assemblies or “inserts.” Each insert typically comprises an insert housing, a ferrule assembly contained within the insert housing and adapted to receive a fiber, and a ball lens at a mating end of the insert housing optically connected to the fiber.
The ball lens as an interface is particularly useful in military targeting. Such an application involves using a fiber to transmit light from a high power optical source (e.g., a laser) to an optical system which images the light onto a target. The ball lens interface is preferred because of the high power density of the light within the fiber. Expanding the beam reduces the power density at the separable interface, which reduces the probability of laser-induced damage caused by interaction of the laser with dirt and debris at the separable interface. In such an application, the laser wavelength is longer than the typical communication wavelength of approximately 1.31 μm to 1.55 μm.
One such ball lens 100 is shown in FIG. 1 (for simplicity only the ball lens 100 and fiber 110 are shown; the insert housing and ferrule assembly of the outer housing are not shown). The ball lens 100 serves to expand and collimate light 101 at the connector interface as shown in FIG. 1. When two expanded beam connectors are mated (shown conceptually in FIG. 2), there is an air gap 220 between the ball lenses 200 of each pair of optically coupled inserts. As in FIG. 1, in FIG. 2 the ball lenses 200 serve to collimate light 102 being carried along fiber 210.
Tyco Electronics Corporation (Harrisburg, Pa.) manufactures expanded beam (EB) connectors using the PRO BEAM® trademark. These expanded beam connectors are for use at typical communications wavelengths such as 0.85 μm, 1.3 μm, 1.31 μm and 1.55 μm, where the first two wavelengths are often used in multimode (MM) applications and the second two wavelengths are used for singlemode (SM) applications. These EB connectors use a 3 mm ball lens to collimate and then refocus the light. A ball lens with an index of refraction of approximately 2 focuses collimated light near the surface of the ball. Therefore in the SM design, the lens material is chosen such that the index of refraction is nearly equal to 2 at 1.31 μM and 1.55 μm. For MM applications, a lens material is used which has an index of refraction less than 2, such that the focal plane is outside of, i.e., external to, the lens. In the MM EB connectors, the mechanical housing (referred to as an “EB insert”) holds the fiber at a predetermined distance from the lens so that the fiber end is positioned at the external focal plane.
A ball lens can be easily manufactured with precise control of the diameter and the sphericity. Therefore the ball lens can be accurately located with respect to a fiber using passive alignment features that are built into the EB insert, using the mechanical characteristics of the ball/sphere shape, i.e., the geometry of a sphere provides a significant “registration surface” for use in positioning of a ball-shaped lens.
The above-described design functions quite well for typical communication wavelengths between 0.85 and 1.55 μm. However, situations arise where a particular application may require connectors that will function with higher wavelengths, such as in the military application described above, or in the near to mid-infrared wavelength range of 1.8 μm to 5 μm. This requires the use of a different glass than is used in the current EB connectors. One glass that is commonly used in infrared optics is zinc selenide (ZnSe), which can pass light in the infrared wavelengths. However, a problem exists when trying to use ZnSe for a ball lens: the index of refraction of ZnSe is greater than 2, and thus a collimated beam of light incident on a ball lens 300 made of ZnSe will focus the light to a point 312 inside of the ball, as illustrated in FIG. 3. Therefore, typical EB lens designs cannot be used as a collimating lens for an EB connector, and yet it is desirable to be able to use the mechanical alignment features of a ball lens. The present invention fulfills this need among others.